The present invention relates to an improvement of a variable displacement vane pump which serves as a hydraulic power source of a hydraulic device such as a power steering apparatus for vehicles.
Japanese Patent Application First Publication No. 2003-74479 discloses a variable displacement vane pump which is applied to a power steering apparatus for vehicles. The variable displacement vane pump of the conventional art includes an adaptor ring fixedly disposed within a pump body, a cam ring which is disposed inside the adaptor ring and swingable about a swing fulcrum on an inner circumferential surface of the adaptor ring, and a rotor which is rotatably disposed inside the cam ring and integrally formed with a drive shaft extending in the pump body. A plurality of radially extending slots are formed at predetermined circumferential intervals in an outer circumferential periphery of the rotor. A plurality of vanes are radially moveably fitted into the respective slots so as to project from the slots and retreat into the slots. Opposed side plates support the cam ring and the rotor therebetween in an axial direction of the rotor. A first fluid pressure chamber and a second fluid pressure chamber are disposed between the adaptor ring and the cam ring in a radially opposed relation to each other. The first fluid pressure chamber is constructed to introduce working fluid having a pressure which is controlled by a control valve. The second fluid pressure chamber is constructed to always introduce a low-pressure fluid from a suction side of the vane pump. This serves for reducing a loss in discharge pressure of the second fluid pressure chamber. The cam ring is swung depending on a relative pressure between the fluid pressure in the first fluid pressure chamber and the spring force of a spring provided on a side of the second fluid pressure chamber. The swing motion of the cam ring causes change in volume of pump chambers each formed between the circumferentially adjacent vanes to thereby control a flow rate of the working fluid discharged by the vane pump. When the vane pump is operated at high speed, the cam ring is swung toward the second fluid pressure chamber to thereby decrease the flow rate of the working-fluid discharged by the vane pump and serve for reducing energy loss thereof.